As next-generation lighting devices or display devices, devices using light-emitting elements (organic EL elements) in which organic compounds are used as light-emitting substances have been developed and commercialized because of their potential for thinness, lightness, high speed response to input signals, low power consumption, and the like.
In an organic EL element, voltage application between electrodes, between which a light-emitting layer is interposed, causes recombination of electrons and holes injected from the electrodes, which brings a light-emitting substance (an organic compound) into an excited state, and the return from the excited state to the ground state is accompanied by light emission. Since the spectrum of light emitted from a light-emitting substance depends on the light-emitting substance, use of different types of organic compounds as light-emitting substances makes it possible to obtain light-emitting elements that exhibit various colors.
For display devices that are expected to display images, such as displays, at least three-color light, i.e., red light, green light, and blue light are necessary to reproduce full-color images. For higher color reproducibility and higher quality of the display images, the color purity of emitted light is increased with the use of a microcavity structure or a color filter.
Furthermore, a variety of measures, such as changing molecular structures of light-emitting materials to be used and adjusting the materials or the compositions of materials of a light-emitting element and the structure of a light-emitting element, are taken to reduce power consumption.